Separating the dispersed phase in a very fine liquid-liquid emulsion or dispersion can be a difficult task. The problem is greatly aggravated if the dispersion is further stabilized due to the presence of electrical charges or by strong interfacial films. This situation is very commonly encountered in the ballast water discharged by tankers carrying crude oil. The problem arises when tankers having discharged their cargo of crude oil, take on ballast, for reasons of stability. Before a new cargo can be taken on, the ballast water contaminated by oil must be discharged.
The oil is dispersed in the ballast water when it is passed through the pumps used for loading or discharging, and the dispersion is stabilized by the presence of strong interfacial films formed by components in the crude. Any chemical aids that would help in flocculating these very fine emulsion systems would play a considerable role in a more complete separation of the oil phase using more conventional techniques.
Ocean waters and the water of harbors connected to the open sea, have a high salinity. These waters also contain a large amount of minerogenic and biogenic suspended material called setson, some of which could play the role of surfactants or flocculants. In addition, harbor waters are quite polluted and could have a large amount of suspended solids due to silt and the discharge from sewers.
U.S. Pat. No. 650,216 teaches a process for the removal of oil from water which comprises the addition of lime to form a soap. It is apparent that this process relates to the removal of vegetable oils and not to the petroleum oils commonly encountered in ballast water treating. This process further teaches that one may, after the addition of lime to precipitate said soap, add a non-crystallizing substance such as a sulfide or hydroxide to help in the filtering of the precipitated soap.
U.S. Pat. No. 3,128,248 teaches a process wherein lime is added to a brine solution to precipitate magnesium hydroxide and calcium carbonate. The precipitate may then be coagulated with oil, soap or polyelectrolyte. There is no teaching, however, that this process may be applied to the separation of oil from water and further this process teaches the equivalency of oil, soap and polyelectrolytes for coagulation of the precipitated materials. The coagulant actually used in the example is a polyelectrolyte. Thus, there is no teaching of the unique advantage of using an oil to efficiently remove a pecipitated phase.
U.S. Pat. No. 3,347,786 teaches a process for the purification of water, wherein said water may be contaminated by a mineral oil, and which comprises the addition of a metallic hydroxide as a flocculating agent to remove suspended matter, redissolving said metal hydroxide, filtering to remove said suspended matter, reprecipitating said metal hydroxide by adjusting the pH, redissolving, etc. This process utilizes repeated precipitating and dissolving steps to remove said suspended matter and does not recognize the advantage of adding an oil to said flocculated salt to remove it.
U.S. Pat. No. 3,301,779 teaches a process for treating cold rolling mill effluent containing oil emulsified in water, which comprises the addition of an alkaline waste, containing sodium hydroxide soap and emulsified oil; and waste pickle liquor, containing sulfuric acid and ferrous sulfate; to said cold rolling mill effluent to form a precipitate, pressurizing said precipitated solution with air, and releasing the pressure, whereby air bubbles are formed which float said precipitated oil to the top where it is recovered for reuse in cold rolling. The teaching of this reference specifically prohibits the use of calcium contaning materials in this process, since it is well known that calcium salts can ruin the properties of oils utilized in cold rolling mill operations.
Finally, Japanese Patent 7224822 teaches a process for separating oil from water which comprises mixing a low specific gravity mineral oil and ferric hydroxide with the oil containing water to form a cotton-like ferric hydroxide, allowing the mixture to stand so that the ferric hydroxide ascends together with contaminated oil droplets and said low specific gravity oil adsorbed on said ferric hydroxide to form an upper layer, separating said upper layer from said aqueous lower layer, adding a paste-like purifying agent comprising ferric hydroxide, mineral oil and a stabilizer such as magnesium oxide to the aqueous layer, allowing the mixture to stand so that the remaining contaminated oil and purifying agent ascend to form an upper layer and subsequently separating upper layer from purified water.